Research in learning and instruction claims a central role for the concept of knowledge. The knowledge base of a person, it is now generally assumed, is made up of different types of knowledge. The most well-known examples are declarative and procedural knovvledge, but more elaborate distinctions exist. Furthermore, the knowledge base is characterized by different qualities, such as level (deep or surface) of knowledge, generality of knowledge, level of autornization of knowledge, modality of knowledge, and structure of knowledge. The present article examines the concept of knowledge by presenting a matrix that takes types arid qualities of knowledge as its dimensions. This matrix can be used to classify research on knowledge by linking aspects such as knowledge assessment techniques, expert-beginner differences, instructional measures, and learning :goals to the cells of the matrix.